The invention relates to a process for assembling a flexible panel on an open, elastically deformable structure and with greater stiffness than the panel. More precisely, the invention relates to a pre-assembly step during which the panel is put into position and is held on the structure before being permanently fixed to it by means of mechanical attachment devices such as rivets.
In the rest of this text, the expression xe2x80x9cflexible panelxe2x80x9d generally denotes any sheet metal element, plate, shell or coating, with a simple or complex structure made from a metallic or other material, and with an arbitrary shape, contour and dimensions.
Furthermore, the expression xe2x80x9copen structurexe2x80x9d denotes any discontinuous, simple or complex structure such as an angle, rail or rib, or a perforated framework formed from several previously assembled elements of this type.
The invention also relates to a pre-assembly installation for use of this process.
The process and the installation according to the invention have a preferred application in the aeronautical industry, and particularly in a flexible workshop for manufacturing aircraft assemblies or subassemblies such as leading edges, or more generally wing or fuselage elements.
In the aeronautical industry, a flexible panel is usually attached to a stiffer structure by means of several mechanical attachment devices such as rivets or bolts that pass through the panel and the adjacent part of the structure that supports it. Therefore, with this assembly type, the panel and the adjacent part of the structure have to be drilled simultaneously at a large number of points before the attachment devices are put into position.
If the assembly is to be made satisfactorily, particularly concerning relative positioning of the panel and the structure, the normal procedure is to use a pre-assembly step before these parts are attached.
Normally, parts are pre-assembled using a xe2x80x9ctemporary fasteningxe2x80x9d technique. With this technique, reference support pins temporarily fix the panel to the open structure in order to achieve correct relative positioning of the two parts during their final attachment.
More precisely, with the temporary fastening technique, it is necessary to firstly make a number of manual preliminary drillings in the panel from the inside of the structure using pilot guide holes in the structure. The diameter of these drillings is less than the diameter of the drillings that will be made later for final attachment of the parts using mechanical attachment devices. The operator then gradually inserts the reference support pins. Counter drillings are then made from the outside at a diameter close to the final diameter, with gradual transfer from the reference support pins. The panel is then disassembled so that it can be trimmed and to remove the swarf. The operators then apply mastic to the structure, then reassemble the panel using pins until the mastic has polymerized.
In order to make the final assembly, the pins are disassembled individually. After each pin has been removed, the parts are drilled or bored to the final required diameter and the attachment device, usually consisting of a rivet, is inserted. The operator then works progressively until each pin has been replaced by a final attachment device.
This traditional assembly technique requires double drilling and several manipulations at each part attachment point. This can lead to assembly inaccuracies and in all cases, requires long working times. This disadvantage is particularly penalizing for a flexible workshop, in which the main objective is to reduce manufacturing costs and times.
Document WO 97/34734 proposes a technique for manufacturing aircraft assemblies or subassemblies such as wings. According to this technique, parts of the assembly to be made are manufactured by numerically controlled machine tools that also make presentation drillings at precise locations in these parts. During the subsequent assembly of the different parts, these drillings are used to directly attach them using traditional attachment devices without any pre-assembly being necessary.
This technique is theoretically attractive, but there are many difficulties with it in practice.
Thus, the predicted assembly precision is only obtained if the presentation drillings made in each part are actually at the required locations. This constraint is difficult to satisfy for parts with complex and relatively flexible parts, which is the case particularly for panels forming skins of wings.
Furthermore, even if this technique eliminates the pre-assembly operation, it is still expensive and takes a relatively long time to implement.
Document FR-A-2 554 878 also describes a known installation and a known process for fixing a metallic skin onto a framework in order to make an aircraft wing. The technique proposed in this document also eliminates all pre-assembly operations.
In this case, the metallic skin is fixed to the framework by hot gluing. This is done by the use of one or several pressurization chambers in a furnace, and an assembly including the framework, the skin and a sealed bag surrounding each part are placed in each pressurization chamber. Rigid plates are placed between the bag and the metallic skin facing the perforated parts in the framework in order to prevent deformation of the skin adjacent to these perforated parts when the intermediate space between the bag and the chamber is pressurized. These rigid plates are supported by a flexible sheet.
This process is not suitable in many cases. Hot gluing requires that a high pressure is applied. When the framework to which the skin is to be fixed is not perfectly rigid, as is almost always the case in the aeronautical industry, application of a pressure in this way would result in unacceptable permanent deformations. This constraint is particularly important when the assembly to be made is not a closed assembly, which is the case particularly for the leading edge of a wing.
Another disadvantage of this technique is that it is very difficult to apply. The relative positioning of the skin and the framework makes it necessary to use guide pins between these two parts and to create a partial vacuum inside the assembly before pressure is applied in the chamber. The positioning of rigid plates facing perforated regions of the framework raises similar difficulties. Therefore, there are good chances that the assembly thus made will not have the required characteristics. This is obviously unacceptable, considering the cost of the assemblies and subassemblies used in the manufacture of an aircraft.
The purpose of the invention is a process for the innovative assembly of a flexible panel on a more rigid structure in order to make pre-assembly of these parts in a much faster manner than is possible using traditional stapling techniques, while enabling an equally precise positioning of parts.
The invention proposes a process for assembly of a flexible panel on an open, elastically deformable structure that is stiffer than the panel consisting of a number of steps in sequence, namely pre-assembly and then attachment of the panel onto the structure using mechanical attachment devices, characterized in that the pre-assembly is done by cold gluing.
The expression xe2x80x9ccold gluingxe2x80x9d as used herein consists of any gluing at essentially ambient temperature and at relatively low pressure compared with the high pressure necessary for hot gluing.
Use of the cold gluing technique for pre-assembly an partly automate this operation and integrate it into a flexible workshop. This operation is about ten times faster than the traditional temporary fastening technique.
Furthermore, cold gluing requires that parts are pressed into contact with each other with a relatively low tightening force. Therefore, there is no risk of permanent deformation of the structure to which the panel is fixed, even it is not a closed structure as in the case of the leading edge of an aircraft wing.
In a preferred embodiment of the invention, cold gluing is done by assembling the structure onto a rigid chassis, by positioning the panel on the structure with insertion of glue and pressing the panel into contact with the structure.
If the structure onto which the panel is to be fixed is a perforated framework, the panel is locally pressed into contact with this framework, mainly in unperforated regions of the framework.
More precisely, when the perforated framework comprises essentially C-shaped ribs and devices forming stiffeners connecting these ribs together, the panel is pressed into contact with these devices over most of their length, and the panel is also pressed into contact with these ribs at local positions between devices forming stiffeners.
When the radius of curvature of the ribs is variable, with a minimum value in the central region, the panel is also pressed into contact with the central regions of the ribs over most of the length of the perforated framework.
Advantageously, the panel is progressively pressed into contact with the perforated framework, firstly by bringing the panels into contact with the said central regions of the ribs and then progressively working towards the rib end regions.
In the preferred embodiment of the invention, the panel is pressed into contact with the perforated structure by means of inflatable joints installed in a rigid gluing cap acting as a mating form.
Advantageously, the panel is then put into position on the chassis supporting the perforated framework without inserting glue, the cap is then moved into the gluing position to hold the panel in position, the cap supporting the panel is moved away from the framework, the glue is applied, and the cap is brought back into the gluing position to press the panel into contact with the framework.
Preferably, the cap is moved into the gluing position by guiding it onto the chassis supporting the framework.
Furthermore, in order to prevent any relative displacement between the cap and the chassis during gluing, it is advantageous to clamp the cap onto the chassis supporting the framework before pressing the panel into contact with the framework.
When the perforated framework comprises at least one overhanging edge, this edge is preferably held in place using thrust devices fitted on the gluing cap before locally pressing the panel into contact with the perforated framework.
In a flexible workshop, it is advantageous to install the structure on the rigid chassis at an assembly station for this structure by assembling its component elements on an assembly template fixed to a frame. The assembly template and the frame then form the rigid chassis. In this case, the rigid chassis supporting the structure is transferred to a gluing station, where the panel is glued to the structure.
The process according to the invention is advantageously applicable to the assembly of a skin forming the flexible panel on a framework forming the structure, when an aircraft leading edge is being manufactured.
The invention also relates to an installation for pre-assembly of a flexible panel on an open, elastically deformable structure with greater stiffness than the panel, before the panel is attached to the structure using mechanical attachment devices, characterized in that it comprises cold gluing means.